In a conventional utility water meter, a mechanical flow transducer (typically positive displacement or single/multi-jet turbine) is coupled to a register mechanism that measures the number of repetitive cycles of the transducer. This mechanism is often a mechanical odometer. To interface this to the electronics required for remote meter reading can be cumbersome and expensive, even when the odometer is replaced by a solid state register having digital counters.
Use of a flow transducer that is also solid-state reduces such interface problems. Such a flow transducer is a magnetic flow transducer of a kind that is well known and shown by way of example in the cross-sectional view of FIG. 1.
Flow tube, 101 incorporates a magnetic transducer 109 comprising a pair of electrodes, 102, disposed across a diameter of the pipe 101, with at least part of one surface of each electrode 102 in intimate contact with the fluid 108 in the pipe. Magnetic pole pieces, 103, are disposed across the orthogonal diameter of the pipe 101 and linked by a magnetic circuit, 104. As is well known in the art, the magnetic field 107 imparts a force on charged species moving with a bulk medium (ions in the case of water), causing the charged species to migrate in a direction orthogonal to both the magnetic field and the direction of bulk fluid motion. The mutual displacement of oppositely charged species results in an electric field along the direction of migration, which builds up until the electrostatic force on a given ion is balanced by the magnetic force. Since the magnetic force depends implicitly on the bulk medium flow velocity, measurement of the opposing electric field (or potential difference) provides a convenient means for determining the flow rate, whilst integration over time allows the total volume that has passed through the tube to be calculated. Circuitry for processing the electrode signals to obtain such measurements is well known in the art and consequently not described in greater detail here.
As is also well known, it can be advantageous to alternate the applied magnetic field, so as to overcome various limitations of a static field measurement. One such limitation is imposed by the nature of the electrodes used to measure the electrical potential difference in the fluid. An ideal electrode will form a perfect electrical connection to the fluid, with no energy barrier to the exchange of charge either way across the solid-liquid interface. This is seldom observed in practical systems, and it is much more likely that an electrical potential difference will be present across the interface. The potential difference is often poorly defined, and varies randomly with time such that it exhibits a noise spectrum that is inversely proportional to frequency (′1/f). A static field (DC) measurement will therefore be subject to large instantaneous errors.
Alternating the applied magnetic field at a known frequency fo partially overcomes this problem: as shown in FIG. 2, this results in the desired electrical signal 201 also being present at the frequency fo which is chosen to be significantly higher than the characteristic frequency of the electrode noise spectrum 202. Measurement of the electrical signal amplitude a provides an indication of the flow rate that is substantially free of errors.
A further reason for applying an alternating magnetic field is that the small-signal electrical impedance of typical electrodes, as perceived by a measuring circuit attached between them, also falls with increasing frequency. The measuring circuit may therefore be permitted to draw more current from the signal source, without causing substantial errors. The principal advantage is that a simpler, cheaper measuring circuit design may be adopted.
To understand the frequency-dependent behaviour of the electrodes, it is useful to consider a simple electrical model of FIG. 3 that is often applied to the solid-liquid interface 301, consisting of a resistor 302 in parallel with a capacitor 303. The direct exchange of charged species between the solid 304 and the liquid 305 is signified by the flow of current through the resistor 302, while the capacitor 303 represents the tendency of charged species to accumulate in the vicinity of the interface, without actually crossing it. At frequencies substantially above 1 Hz, the capacitor 303 generally provides the easier route for the flow of a small-signal current through a solid-liquid interface.
In the device of FIG. 1, an alternating magnetic field is achieved by means of coils 105 wound around part of magnetic circuit 104 and supplied with a suitable alternating current waveform. Furthermore, to reduce power consumption, it is known to provide magnetic circuit 104 with one or more elements 106 exhibiting magnetic remenance so that the coils need only be energised when it is required to change the state of the magnetic field.
The present invention has as an objective yet further reduction in the power consumption of magnetic flow transducers.